The Compton-thick AGN population and the NH distribution of low-mass AGN in our cosmic backyard

Year: 2025 DOI: 10.1093/mnras/staf956
We present a census of the Compton-thick (CT) active galactic nucleus (AGN) population and the column density (N_H⁠⁠) distribution of AGN in our cosmic backyard using a mid-infrared selected AGN sample within 15 Mpc. The column densities are measured from broad-band X-ray spectral analysis, mainly using data from Chandra and NuSTAR. Our sample probes AGN with intrinsic 2–10 keV luminosities of L₂₋₁₀,int = 10³⁷-10⁴³ erg s⁻¹, reaching a parameter space inaccessible to more distant samples. We directly measure a 32⁺³⁰₋₁₈ CT AGN fraction and obtain an N_H distribution that agrees with that inferred by the Swift-BAT survey. Restricting the sample to the largely unexplored domain of low-luminosity AGN with L₂₋₁₀,int ≤ 10⁴² erg s⁻¹⁠, we found a CT fraction of ⁠19⁺³⁰₋₁₄ per cent, consistent with those observed at higher luminosities. Comparing the host-galaxy properties between the two samples, we find consistent star formation rates, though the majority of our galaxy have lower stellar masses (by ≈ 0.3 dex). In contrast, the two samples have very different black hole mass (M_(BH)⁠⁠) distributions, with our sample having ≈ 1.5 dex lower mean mass (M_(BH)⁠ ~10⁶ M⊙⁠). Additionally, our sample contains a significantly higher number of LINERs and H ii-type nuclei. The Eddington ratio range probed by our sample, however, is the same as Swift-BAT, although the latter dominates at higher accretion rates, and our sample is more evenly distributed. The majority of our sample with λEdd ≥ 10⁻³ tend to be CT, while those with λEdd < 10⁻³ are mostly unobscured or mildly obscured.